Noisy depth of field
DOF raytracing requires more render samples, because the same pixel requires more rays to be calculated.

Imagine simple pin-hole camera with no DOF - rays are shot from camera pixels and each lands on specific point in the scene. After that they scatter (that's why you need many samples per pixel).

Now with DOF the aperture is open so every ray from each pixel has more ways to exit the camera (infinitely more) and can land on different points in the scene. So the bigger the aperture the more samples you need to get clear image.